A scorotron charger including a grid is known as a corona charger that charges a photosensitive member, which is a member to be charged. There are basically two types of grids. One type is a wire grid that includes wires arranged in an opening in a housing of a corona charger so as to extend in a longitudinal direction. The other type is an etching grid produced by forming many holes in a thin plate by an etching process.
The etching grid covers a larger area of the opening in the housing (has a smaller opening ratio) than the wire grid, and is therefore advantageous in that the potential of a photosensitive member can be easily controlled to a target potential. However, the etching grid more easily allows discharge products generated in a discharge process to adhere thereto than the wire grid.
The discharge products that have adhered to the etching grid (hereinafter referred to as grid) accelerate oxidation of the grid, and rusted parts of the grid have charging characteristics different from those of the other parts. This leads to non-uniform charging. PTL 1 discloses a structure for suppressing non-uniform charging by forming a protective layer, which mainly contains carbon atoms in an SP3 structure, on a surface of a base member of the grid and increasing corrosiveness against the discharge products. PTL 1 also discloses a structure in which the thickness of a protective layer that covers a front surface of the grid that faces a discharge electrode is greater than the thickness of a protective layer that covers a back surface of the grid since the surface of the grid that faces the discharge electrode is easily corroded by the discharge products.
PTL 2 discloses a structure including a cleaning member for removing toner and discharge products (hereinafter referred to as foreign matters) that have adhered to a grid. Specifically, a cleaning brush that serves as the cleaning member is provided to clean the grid from a discharge-electrode side of the grid while both end portions (edge portions) of the grid in a lateral direction are pressed against the cleaning brush at a side of the grid that faces a member to be charged.
As a result of studies conducted by the inventors, it has been found that non-uniform charging occurs in a relatively short time when both end portions of a grid in a lateral direction are pressed against a cleaning member at a side facing a member to be charged as in PTL 2.
This is probably because in the structure in which the grid is held at both ends thereof and pressed against the cleaning member to stabilize the amount of contact intrusion by which the cleaning member intrudes into the grid, the protective layer that covers the base member of the grid wears in local contact regions. The local wear results in the base member becoming exposed, and the base member becomes rusted from the exposed portions thereof. This is probably the cause of non-uniform charging.